Wet oxidation of sewage sludge: full-scale experience and process modeling

In situ remediation of oil-contaminated soils by ozonation: Experimental study and numerical modeling

The goal of the present work is to quantify the performance of ozonation as a method for the in situ remediation of soils polluted at varying degree with different types of hydrocarbons, and assess its applicability, in terms of remediation efficiency, cost factors, and environmental impacts. Ozonation tests are conducted on dry soil beds, for three specific cases: sandy soil contaminated with low, moderate and high concentration of a non-aqueous phase liquid (NAPL) consisting of equal concentrations of n-decane, n-dodecane, and n-hexadecane; sandy soil polluted with diesel fuel; oil-drilling cuttings (ODC). The transient changes of the concentration of the total organic carbon (TOC), total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), and soluble chemical oxygen demand (SCOD) in soil and carbon dioxide (CO2), carbon monoxide (CO), volatile organic compounds (VOCs), and ozone (O3) in exhaust gases are recorded. A numerical model is suggested where the ozone is adsorbed on solid grains, reacts with adsorbed organic species, and desorbed CO2, CO along with unconsumed O3 are released. The model is used to estimate the ozonation kinetic parameters, upscale the process, and estimate the cost and gas emissions per 1 tn of treated soil. Experiments reveal that after 4h of treatment, the TOC decreases profoundly only for the low NAPL concentration (76.4%), the highest and fastest TPH removal efficiency occurs for the moderate NAPL concentration (∼88%), and diesel fuel (87%), while the TPH removal efficiency becomes high enough for oil-drilling cuttings (80%) after 8h of treatment. In all cases, mainly CO2 is detected in exhaust gases, its cumulative mass is fully consistent with the TOC losses, while the O3 consumption is enhanced for heavily polluted soils. The concentration of PAHs is reduced profoundly for both the diesel fuel and ODC. The lowest energy consumption per unit mass of degraded TPH or TOC occurs for the heavily polluted soils. The cost of soil treatment increases with the initial TPH concentration and pollutant complexity increasing.

Aging and mitigation of microplastics during sewage sludge treatments: An overview

Wastewater treatment plants (WWTPs) receive large quantities of microplastics (MPs) from raw wastewater, but many MPs are trapped in the sludge. Land application of sludge is a significant source of MP pollution. Existing reviews have summarized the analysis methods of MPs in sludge and the effect of MPs on sludge treatments. However, MP aging and mitigation during sludge treatment processes are not fully reviewed. Treatment processes used to remove water, pathogenic microorganisms, and other pollutants in sewage sludge also cause surface changes and degradation in the sludge MPs, affecting the potential risk of MPs. This study integrates MP abundance and distribution in sludge and their aging and mitigation characteristics during sludge treatment processes. The abundance, composition, and distribution of sludge MPs vary significantly with WWTPs. Furthermore, MPs exhibit variable degrees of aging, including rough surfaces, enhanced adsorption potentials for pollutants, and increased leaching behavior. Various sludge treatment processes further intensify these aging characteristics. Some sludge treatments, such as hydrothermal treatment, have efficiently removed MPs from sewage sludge. It is crucial to understand the potential risk of MP aging in sludge and the degradation properties of the MP-derived products from MP degradation in-depth and develop novel MP mitigation strategies in sludge, such as combining hydrothermal treatment and biological processes.

Process modelling and techno-economic analysis of anaerobic digestion of sewage sludge integrated with wet oxidation using a gravity pressure vessel and thermal hydrolysis

Anaerobic digestion (AD) of sewage sludge is used to biodegrade sewage sludge into biomethane and digestate. With the addition of thermal processes such as thermal hydrolysis (TH) and wet oxidation (WO), AD biodegradability generally improves. Implementation of additional treatment is challenging due to the limitation in the mass and energy balances. Hence, tools such as process simulation can be utilized to predict the input and output around the process. In addition, an economic analysis needs to be conducted to check the economic feasibility. The techno-economic analysis (TEA), an integrated method to evaluate a process scheme through simulation and subsequent economic analysis, is effective in providing a systematic understanding of economic implications and the feasibility of a process by identifying the bottlenecks and uncertainties that have a significant impact on the technology. TEA of AD, especially incorporating the TH or WO using gravity pressure vessel (GPV) technology, is limited in the literature. A comprehensive TEA of the AD and the pre- and post-treatment schemes can be utilized to determine the most feasible pathway for sludge treatment for implementation in the wastewater industry. In this study, TEA for four different scenarios of AD was conducted using Aspen Plus and economic analysis tools: (1) without any pre- or post-treatment, (2) with TH pre-treatment, (3) with 100 % WO post-treatment, and (4) with 20 % partial wet oxidation (PWO) and acid hydrolysis pre- or post-treatment. A simulation model (GPVM) was developed using Aspen Plus to mimic the GPV reactor. The study outcomes showed that Scenario 3 with 100 % WO post-treatment was the most suitable for processing parameters and sludge treatment cost. The sensitivity analysis concluded that operating cost and plant capacity are the dominant factors that impact the plant feasibility significantly.

Effects of reaction conditions on the wet oxidation of excess sludge from the caprolactam wastewater treatment process

In this study, wet oxidation of excess sludge from the caprolactam wastewater treatment process was investigated. The effects of reaction conditions, including sludge concentration, the amount of sludge, reaction temperature and time and the oxygen supply amount, were discussed. The highest removal rates of chemical oxygen demand (COD) and volatile suspended solid (VSS) that can be attained at 78.6 and 89.3% were acquired separately under 260 °C for 60 min with an initial oxygen gas pressure of 1.3 MPa. The sludge was hydrolyzed and oxidized under hydrothermal conditions, producing small molecule organic acids, such as acetic, formic and oxalic acids, as the main products, which could be used as carbon sources for wastewater treatment. These results indicated that wet oxidation provides a favourable and feasible alternative method for the treatment of excess sludge from the coal chemical industry.

Advanced thermal hydrolysis for biopolymer production from waste activated sludge: Kinetics and fingerprints

Waste activated sludge (WAS) is the main residue of wastewater treatment plants, which can be considered an environmental problem of prime concern due to its increasing generation. In this study, a non-energetic approach was evaluated in order to use WAS as a renewable resource of high value-added products. For this reason, WAS was treated by thermal hydrolysis, H₂O₂ oxidation and advanced thermal hydrolysis (ATH) promoted by H₂O₂. The influence of temperature, H₂O₂ concentration and dosing strategy on biomolecule production (proteins and carbohydrates), size distribution (fingerprints) and various physico-chemical parameters (VSS, total and soluble COD, soluble TOC, pH and colour) was studied. The results revealed a synergistic effect between TH and H₂O₂ oxidation, which led to a significant increase in the production of both proteins and carbohydrates. In this sense, the concentration of proteins and carbohydrates obtained during TH at 85 °C for120 min was found to be 1376 ± 9 mg/L (121 mg/gVSS_o) and 208 ± 4 mg/L (18 mg/gVSS_o), respectively. However, in the presence of 4.5 mM H₂O₂/gVSS_o under the same process conditions, the concentrations of proteins and carbohydrates exhibited a significant increase of 1.9-fold and 3.1-fold, respectively. Besides, the addition of H₂O₂ promoted the transformation of hydrophobic compounds, such as proteins and or lipids, into hydrophilic compounds, which presented low and medium sizes. An increase in temperature improved the solubilization rate and the yield of biomolecules significantly. Besides, the analysis of the kinetics related to the dosing strategy of H₂O₂ suggested the existence of two fractions during WAS solubilization, one of them being easily oxidizable, whereas the other one was more refractory to oxidation. Thus, the value of k_H2O2 for the first addition of 1 mM H₂O₂/g VSS_o was 0.020 L^0.4 mgH₂O₂^-0.4 min^-1, while it was 4.3 and 8 times lower for the second and third additions, respectively.

Life cycle sustainability assessment of advanced treatment techniques for urban wastewater reuse and sewage sludge resource recovery

Wastewater treatment plants can become a source of valuable resources, such as clean water, energy, fuels and nutrients and thus contribute to the sustainable development goals and a transition to a circular economy. This can be achieved by adopting advanced wastewater and sludge treatment techniques. However, these have to be evaluated on their sustainability to avoid any unintentional consequences. Therefore, this paper presents a life cycle sustainability assessment of advanced wastewater and sludge treatment techniques by integrating the environmental, economic and social aspects. The options considered for advanced wastewater treatment are: i) granular activated carbon; ii) nanofiltration; iii) solar photo-Fenton; and iv) ozonation. The technologies for advanced sludge treatment are: i) agricultural application of anaerobically digested sludge; ii) agricultural application of composted sludge; iii) incineration; iv) pyrolysis; and v) wet air oxidation. The results for the advanced wastewater treatment techniques demonstrate that nanofiltration is the most sustainable option if all the sustainability aspects are considered equally important. If, however, a higher preference is given to the economic aspect, ozonation and granular activated carbon would both be comparable to nanofiltration; if the social aspect is considered more important, only activated carbon would be comparable to nanofiltration. Among the sludge treatment methods, agricultural application of sludge is the most sustainable technique for mean-to-high resource recovery. If the recovery rate is lower, this option is comparable with incineration and pyrolysis with high recovery of their respective products. This work helps to identify the most sustainable techniques that could be combined with conventional wastewater treatments for promoting wastewater reuse and resource recovery across a wide range of operating parameters and products outputs. The findings also support the notion that more sustainable wastewater treatment could be achieved by a circular use of water, energy and nutrients contained in urban wastewaters.

Biosolids: The Trojan horse or the beautiful Helen for soil fertilization?

The simultaneous requirement to manage resources and wastes in more rational way has meant that many communities worldwide have begun to search for long-term alternative solutions. Reuse and recovery of biosolids is considered to be a constant solution of circular sustainability, as waste disposal without further reuse background like fertilizer is no longer an alternative to be promoted. There have been developed many treatment methods over the years for the stabilization and sanitization of biosolids. However, the literature concludes that none of them is fully integrated by meeting all the basic criteria. Each method has its Achilles heel, and the appropriateness of the method lies in what is the goal each time. There are conventional methods with positive reciprocity in terms of sustainability, reuse indicators and technological maturity, but have high risk of microorganisms' reappearance. New advanced sustainable technologies, such as cold plasma, need to be further studied to apply on a large scale. The reuse of biosolids as construction materials is also discussed in the context of circular economy. Biosolids reuse and management legislation frame need to be revised, as a directive adopted 30 years ago does not fully meet communities' current needs.

AOPs-based remediation of petroleum hydrocarbons-contaminated soils: Efficiency, influencing factors and environmental impacts

Petroleum hydrocarbons are a class of anthropogenic compounds including alkanes, aromatic hydrocarbons, resins, asphaltenes and other organic matters, and soil pollution caused by petroleum hydrocarbons has drawn increasing interest in recent years. Multiple advanced oxidation processes (AOPs) are emerging to remediate petroleum hydrocarbons-contaminated soils, while very few studies have focused on the features of AOPs applied in soils. This review aims to provide an updated overview of the state of the science about the efficiency, influencing factors and environmental implications of AOPs. The key findings from this review include: 1) cyclodextrin and its derivatives can be used to synthesize targeting reagents; 2) soil organic matter (SOM), glucose and cement can activate persulfate; 3) SOM affects redox circumstance in soil and could be further developed for enhancing the catalysis effect of transition metals; 4) non-thermal plasma and wet oxidation are promising methods of AOPs to remove petroleum hydrocarbons from soil; 5) the occurrence, fate, and transformation of intermediates during the degradation of petroleum hydrocarbons in soil should be considered more. Overall, this review reveals an urgent need to develop the cost-effective remedial strategies for petroleum hydrocarbons contaminated soils, and to advance our knowledge on the generation, transport and propagation of radicals in soils.

What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?

Similar to other types of waste, sewage sludge (SS) must be minimized, not only to respect the European Directive 2018/851 on waste, but also because the cost of sludge management is approximately 50% of the total running costs of a wastewater treatment plant (WWTP). Usually, minimization technologies can involve sewage sludge production with three different strategies: (i) adopting a process in the water line that reduces the production of sludge; (ii) reducing the water content (dewatering processes) or (iii) reducing the fraction of volatile solids (stabilization). This review, based on more than 130 papers, aims to provide essential information on the process, such as the advantages, the drawbacks and the results of their application. Moreover, significant information on the technologies still under development is provided. Finally, this review reports a discussion on the impact of the application of the proposed processes in the sludge line on a WWTP with a capacity exceeding 100,000 population equivalent (PE).

Performance of non-catalytic thermal hydrolysis and wet oxidation for sewage sludge degradation under moderate operating conditions

Land disposal of waste activated sludge pose environmental risks due to the presence of heavy metals, pathogens and organic pollutants. Anaerobic digestion (AD) is one of the preferred treatment methods for sludge treatment. However, sludge hydrolysis is often found the rate-limiting step thereby reducing the biogas generation potential. Therefore, an effort was made to determine the optimum conditions for sludge solubilization by means of hydrothermal pretreatment. In this study, response surface methodology using Box Behnken design approach was used to optimize four hydrothermal reaction variables (i.e., initial pH, time, temperature, and oxidation coefficient) for sludge solubilization and total chemical oxygen demand (tCOD) reduction. Temperature and pH were found to be the most significant parameters. The maximum tCOD reduction and volatile suspended solids solubilization of 58% and 52%, respectively, were obtained at the following optimum conditions: temperature = 180 °C, time = 5 h, pH = 3.3 and oxidation coefficient = 0.5. Under similar conditions and alkaline pH, Maillard's reaction occurs which may have adverse impact on the performance of downstream AD process. The highest NH₄⁺N and volatile fatty acids (VFAs) concentrations were detected in the treated sludge at the optimum conditions. The future studies should be aimed at the recovery of proteins, VFAs and biogas using appropriate methods.

Environmental comparison of alternative treatments for sewage sludge: An Italian case study

A Life Cycle Assessment (LCA) was applied to compare different alternatives for sewage sludge treatment: such as land spreading, composting, incineration, landfill and wet oxidation. The LCA system boundaries include mechanical dewatering, the alternative treatment, transport, and final disposal/recovery of residues. Cases of recovered materials produced as outputs from the systems, were resolved by expanding the system boundaries to include avoided primary productions. The impact assessment was calculated using the CML-IA baseline method. Results showed that the incineration of sewage sludge with electricity production and solid residues recovery collects the lowest impact indicator values in the categories human toxicity, fresh water aquatic ecotoxicity, acidification and eutrophication, while it has the highest values for the categories global warming and ozone layer depletion. Land spreading has the lowest values for the categories abiotic depletion, fossil fuel depletion, global warming, ozone layer depletion and photochemical oxidation, while it collects the highest values for terrestrial ecotoxicity and eutrophication. Wet oxidation has just one of the best indicators (terrestrial ecotoxicity) and three of the worst ones (abiotic depletion, human toxicity and fresh water aquatic ecotoxicity). Composting process shows intermediate results. Landfill has the worst performances in global warming, photochemical oxidation and acidification. Results indicate that if the aim is to reduce the effect of the common practice of sludge land spreading on human and ecosystem toxicity, on acidification and on eutrophication, incineration with energy recovery would clearly improve the environmental performance of those indicators, but an increase in resource depletion and global warming is unavoidable. However, these conclusions are strictly linked to the effective recovery of solid residues from incineration, as the results are shown to be very sensitive with respect to this assumption. Similarly, the quality of the wet oxidation process residues plays an important role in defining the impact of this treatment.

Fundamental mechanisms and reactions in non-catalytic subcritical hydrothermal processes: A review

The management and disposal of solid waste is of increasing concern across the globe. Hydrothermal processing of sludge has been suggested as a promising solution to deal with the considerable amounts of sludge produced worldwide. Such a process not only degrades organic compounds and reduces waste volume, but also provides an opportunity to recover valuable substances. Hydrothermal processing comprises two main sub-processes: wet oxidation (WO) and thermal hydrolysis (TH), in which the formation of various free radicals results in the production of different intermediates. Volatile fatty acids (VFAs), especially acetic acid, are usually the main intermediates which remain as a by-product of the process. This paper aims to review the fundamental mechanism for hydrothermal processing of sludge, and the formation of different free radicals and intermediates therein. In addition, the proposed kinetic models for the two processes (WO and TH) from the literature are reviewed and the advantages and disadvantages of each model are outlined. The effect of mass transfer as a critical component of the design and development of the processes, which has been neglected in most of these proposed models, is also reviewed, and the effect of influencing parameters on the processes' controlling step (reaction or mass transfer) is discussed.